Gallery and print store

Friday, 22 August 2014

The Triassic ornithodiran Scleromochlus taylori depicted as a nocturnal desert-specialist with filamentous insulation, fuzzy feet for purchase on drifting sands and a saltatorial means of locomotion.

Like actors with one famous character, fossil taxa can become typecast to specific ‘roles’ in palaeontological discussions. One fact of their palaeobiological significance is entrenched so deeply that they are seldom mentioned outside of this context. Examples include Archaeopteryx as the first bird, Mei as the cute sleeping dinosaur, and Darwinopterus as the bridge between major stages of pterosaur evolution. Packaging these animals into simple factoids obscures much of their other interesting palaeobiology, so we rarely hear about their other remarkable features.

Step forth Scleromochlus taylori, a small Triassic archosaur from the Upper Triassic (Late Carnian) of Scotland. For 100 years Scleromochlus has been implicated as a relative of pterosaurs (e.g. Huene 1914; Padian 1984; Gauthier 1986; Sereno 1991; Bennett 1996; Hone and Benton 2008; Brusatte et al. 2010, Nesbitt 2010) or, at very least, an ornithodiran representing a very early stage of stem-bird evolution (Benton 1999; Hone and Benton 2008).* This is about all we ever hear about Scleromochlus, however: nothing more than a milestone in the evolution of pterosaurs or dinosaurs. I'm guilty of it too: in my own book, Pterosaurs (Witton 2013), Scleromochlus just formed an anchor for discussing ornithodiran evolution. Undoubtedly, this needs correcting: Scleromochlus is a unique and interesting animal in its own right, and one fully worthy of detailed discussion. To relieve my shame, I'm going to attempt such a discussion here. Just for fun, I'm going to write it in the same style as a Pterosaurs chapter.

*You can't mention Scleromochlus on the internet without someone pointing out that its status as an ornithodrian has not been tested in analyses containing non-archosaur archosauromorphs. This is true enough, but - at least within the current limits of testing - its ornithodiran status is not controversial, having been recovered in at least six different analyses (e.g. Gauthier 1986; Sereno 1991; Bennett 1996; Hone and Benton 2008; Brusatte et al. 2010) and sharing several unique characteristics with Pterosauria (Padian 1984). Hence, we're following convention here.

Select line drawings of Scleromochlus taylori fossils from Benton (1999). There are two specimens here, showing dorsal and ventral views. The specimen on the right is the holotype, and the left shows two associated individuals. Note the banded scales crossing the vertebrae of the larger individual.

Although represented by at least seven specimens from the Lossiemouth Sandstone Formation, no Scleromochlus is well preserved (Benton 1999). Most specimens comprise shallow sediment molds rather than actual bones, and none are complete. But we should consider ourselves lucky we know of this animal at all: the delicate, 180 mm long bodies of Scleromochlus occur in sandstone deposits representing an ancient, wind-blown desert with 20 m high dunes. Such deposits are often devoid of fossil remains, but the Lossiemouth Sandstones actually preserve a diverse reptile fauna (Benton and Walker 1985). Still, it’s remarkable that the tiny bones of these reptiles preserved at all in these harsh conditions and in relatively coarse (fine - medium) sands - the grains preserving Scleromochlus are each as large as Scleromochlus teeth. As is typical of Lossiemouth Sandstone specimens, most Scleromochlus fossils are more-or-less articulated and many appear to have been crouching at death. With little indication of sun-cracking or scavenging, their remains clearly represent animals which were buried alive or buried shortly after death, probably by sandstorms or dune collapses (Benton and Walker 1985). Although likely complete when buried, no specimens have survived intact to the present. Cross-scaling elements from different specimens has permitted a reasonable insight into Scleromochlus anatomy all the same (Benton 1999). Some details remain murky however, and disagreement persists over precise bone lengths and skull bone attitudes (Sereno 1991, Benton 1999; Padian 2008). This is perhaps expected, given that Scleromochlus remains are interpreted via low-angle light and plaster or plastic peels of the skeleton molds. Bennett (1996) sums up working on Scleromochlus as "low-angle illumination [is used] to examine and interpret molds and peels, but in my experience a considerable amount of imagination is necessary as well".

Anatomy

Specific details aside, palaeontologists are happy to say that the basic bauplan of Scleromochlus resembles a small lizard with enormous hindlimbs (below). The skull has a low lateral profile but is rather triangular in dorsal aspect, with a blunt muzzle and widened posterior. So far as can be seen, the orbit is by far the largest opening in the skull, making the reduced nares look even smaller by comparison. The temporal fenestrae - as illustrated by Benton (1999) - are fairly sized, although their full margins aren't clear in any specimen. These sit above a posteriorly lengthened retroarticular process on an otherwise fairly unremarkable lower jaw. Each jaw seems to house 15/16 teeth, which are apparently isodont and - so far as can be seen - relatively small and lanceolate. The lizard-like visage of Scleromochlus is further enhanced by its short neck, which contrasts with later ornithodirans. The tail appears rather short too, being about as long as the snout-vent length.

Reconstructed skeleton of Scleromochlus taylori from Witton (2013), a modified version of the skeletal in Benton (1999).

The limbs of Scleromochlus are where a lizard-like visage starts to unstick. The forelimb bones are long and slender, and capped with tiny hands. The fingers are poorly known, but the tiny metacarpals suggest they were rather diminutive and unlikely of any use for standing or walking, a hypothesis supported by the dichotomy in fore- and hindlimb length. Even less lizard-like are the hindlimbs, which are extremely long - about half the length of the entire animal - and end with a narrow foot with tightly bound metatarsals. Both the forelimbs and pelvis appear relatively small compared to the legs, though neither is atypically small for the length of the animal. The fifth toe appears to have been lost, the only remnant being a short, pointed metatarsal. Scleromochlus hindlimb arthrology betrays a parasagittal posture akin to that of dinosaurs and pterosaurs - the suite of characteristics associated with this is one clue that Scleromochlus is closely related to these clades (Bennett 1996; Benton 1999; Hone and Benton 2008).

Thin, transversely-banded scutes(?) covered the dorsal surface of the Scleromochlus torso, extending from at least the shoulders to the posterior pelvic region (indicated in the fossil illustrations, above). These indicate that Scleromochlus was at least partly scaled, although whether this represents the entire integument is not clear. It is increasingly apparent that scraps of fossil skin do not tell whole stories about ornithodiran integuments, as more and more specimens with extensive skin preservation present 'mosaics' of scales, naked skin and various kinds of filaments (demonstrated in pterosaurs, theropods and ornithischians; e.g. Bakhurina and Unwin 1994; Chiappe and Göhlich 2010; Godefroit et al. 2014). Scleromochlus may have been covered in scales, but it is equally likely that it had fuzz-like filaments in places. There are several reasons for this. Firstly, it belongs within a phylogenetic bracket where filaments are the ancestral condition or, at very least, scales were prone to developing filamentous morphologies. Secondly, virtually all models of archosaur evolution recover Scleromochlus as sister taxon to a fuzzy clade - pterosaurs, so there is good 'phylogenetic proximity' for fuzz. Thirdly, insulating integuments are common - if not ubiquitous - in small, active (see below) desert-dwelling animals. Thus, while the overall integument of Scleromochlus remains mysterious, a mosaic of filaments and scales is not an unreasonable suggestion. In the reconstruction here, Scleromochlus is shown as rather fuzzy all over (see below for rationale), with filaments poking through its scaly back as they do on opossum tails and armadillo hide.

Please provide your own 'boing' sound effects.

Locomotion

Scleromochlus has long been recognised as a sprightly, cursorial or saltatorial biped because of its elongate, parasagittal hindlimbs (e.g. Woodward 1907; Huene 1914, Walker 1961; Padian 1984; Benton and Walker 1985; Benton 1999; Witton 2013). It has also been considered an arboreal glider with Sharovipteryx-like hindlimb membranes, as well as an aquatic diver, but few obvious adaptations to these lifestyles are found on its skeleton (Benton and Walker 1985). Cursorial features of Scleromochlus include lengthening of the distal hindlimb, reduction of the lateral pedal digits and narrowing of the metatarsal, and it is generally considered to have assumed a digitigrade stance, at least when moving at speed. Several features indicate that Scleromochlus was a saltator rather than a running creature: a relatively small but strong pelvis, short trunk skeleton, and a pronounced intercondylar groove at the end of the femur, which likely reflects a large quadriceps femoris tendon (Benton and Walker 1985). Saltation is an energy-efficient means of locomotion which has frequently evolved in desert-living species - extant examples include desert rodents, jerboas and kangaroos - and Scleromochlus has been favourably compared with such animals on several occasions (Walker 1961; Benton and Walker 1985; Benton 1999). Saltation may seem unusual means for a reptile to move, but other Triassic ornithodirans may have also locomoted in this way (Sereno and Arcucci 1994). Indeed, the powerful leaping and bounding abilities of early ornithodirans has been tied to the evolution of pterosaur flight (Bennett 1997; Witton 2013).

Lifestyle and palaeoecology

It is difficult to say exactly what Scleromochlus ate because its teeth are poorly known, but a generalised diet of insects and other small prey seems mostly likely given the shape of its teeth and jaws (Benton and Walker 1985; Benton 1999). The wide skull and enlarged retroarticular process may have provided space for large and powerful jaw muscles, allowing Scleromochlus to make short work of tough insect carapaces. The association of crouching, articulated Scleromochlus skeletons (see line drawings, above), along with the recovery of multiple specimens (actually 5% of the Lossiemouth Sandstone fauna - Benton and Walker 1985), hints at some degree gregarious behaviour. It is difficult to imagine how the two associated individuals shown above were preserved in such a way unless they were alongside each other when they died - huddled together against whatever catastrophe buried them. Was coupling or group living 'normal' behaviour in Scleromochlus? Statistically, the odds of rare fossil specimens like those of Scleromochlus preserving unusual, 'one in a million' types of behaviour are low, so we might take the co-preservation of two animals as being representative of 'average' or typical behaviour in this species.

For those of you now weeping about tiny, panicked pairs of Scleromochlus dying in huddled balls of fear, here's a speculative reconstruction baby Scleromochlus to cheer you up. Using back of the envelope calculations of lizard egg mass and size, I predict this gangly hatchling was 50-60 mm long. The image is deliberately displayed at this size to stress the tiny proportions: on my 'standard issue' laptop screen, it's about life-size. Click to embiggen.

The likely saltatorial locomotion of Scleromochlus may not be their only adaptation to desert life. Their metatarals are rather flattened posteriorly (Benton and Walker 1985), permitting sitting or squatting on plantigrade feet without sinking into sand. Their nares are small, and flanges from the back of the skull cover the tympanic region (the location of the ear opening), both adaptations common among modern xerocoles to prevent moisture loss and minimise irritation from wind-blown sands (Benton and Walker 1985). Their orbits, by contrast, are very large, and may reflect another common response to desert life - nocturnality. Tiny animals like Scleromochlus rapidly overheat under a desert sun, but foraging at night negates that risk. Of course, desert temperatures plummet once the sun sets, but a layer of filaments (if present) may have countered this. Perhaps groups of Scleromochlus spent their days under shelter - rocks or vegetation - before venturing out at night to forage for insects. This strategy also helps avoid predators, of which the Lossiemouth Sandstone Formation has its fair share: early dinosaurs and nimble pseudosuchians are likely predators of Scleromochlus (Benton and Walker 1985).Hypothetical filaments of Scleromochlus may have had further uses in desert life, including enhancing their grip - and therefore agility - on sandy substrates, as seen in some modern saltatorial desert species. Likewise, covering or filling nose and ear openings with long scales or fur is another feature common to desert species, enhancing resistance to evaporation and airborne sand. The desert habitat of this early ornithodiran presents several intriguing reasons for the development of filamentous structures, which is obviously of interest when considering the origins of fuzz in Ornithodira more broadly.

And that, in a way, brings us full circle: back to considering Scleromochlus anatomy in the context of wider Ornithodira. Still, I'm sure we can all agree Scleromochlus is actually a very interesting animal in its own right, and definitely worthy of escaping typecasting as 'the early ornithodiran'.

Gauthier, J. A. (1986). Saurischian monophyly and the origin of birds. In Padian, K. The Origin of Birds and the Evolution of Flight, Memoirs of the California Academy of Sciences 8. California Academy of Sciences, 1–55.

Walker, A. D. (1961). Triassic reptiles from the Elgin area: Stagonolepis, Dasygnathus and their allies. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 103-204.

Wednesday, 13 August 2014

Yesterday, a huge team of authors called out the science behind 'Thalassodromeus sebesensis', an alleged new pterosaur species 40 million years and thousands of miles out of time and space (Grellet-Tinner and Codrea 2014). As with many outlandish palaeontological claims, the evidence behind 'T. sebesensis' really falls apart rapidly under scrutiny, principally because the alleged pterosaur remains actually represent an unremarkable piece of turtle plastron (Dyke et al. 2014).

A, the plastron of the fossil Romanian turtle Kallokibotion magnificum, compared with B, the alleged holotype 'cranial crest' of 'Thalassodromeus sebesensis'. For further details, see yesterday's post.

Since then, the response to our comment has been published (Codrea and Grellet-Tinner 2014). I'll admit to being surprised that Codrea and Grellet-Tinner maintain the specimen as a pterosaur, and consider the arguments raised against our points as weak, hypocritical and problematic, but whatever: the two arguments are now out, and the palaeontological community can judge for themselves. CT scanning is apparently planned for the specimen (Codrea and Grellet-Tinner 2014), which should put 'T. sebesensis' to bed once and for all.

This post isn't really about that, though: it's about correcting a mistruth in Codrea and Grellet-Tinner's response. Their comment shows little decorum or professionalism, attempting to undermine our response with ad hominem potshots at some authors of Dyke et al. (2014), including criticism of their editorial skills and the taxonomic confusion surrounding specimens described by the authors. Moreover, they criticise us for not examining the specimen, UBB ODA-28, before publishing our response. They state that:

"...UBB ODA-28 is housed in an official and recognized Romanian institution, thus available for examinations to anyone interested. This includes Dyke’s July 2nd 2014 written request to examine UBB ODA-28, which was immediately granted, although, Dyke went on writing its hasty comment without examining UBB ODA-28."

Codrea and Grellet-Tinner, 2014, p. 3-4 (my emphasis)

Well, this isn't really true. Some of it is: Gareth Dyke did write to ask for permission to look at the specimen this year - specifically between July and September - but 'immediate' access was not granted. Rather, eventual access was promised following on-going studies, including CT scanning of the specimen, the dates of which was not disclosed. This is not, as Codrea and Grellet-Tinner describe, 'immediately' granting access, but nebulously promising access at an undetermined future date.

This may not seem like a big deal, but our integrity is being questioned for having not seen the specimen, so we - the authors of Dyke et al. (2014) - think the record should be set straight. There's no doubt that examining specimens is the way forward in any research. But it was clear from Gareth's correspondence that accessing UBB ODA-28 was going to be difficult for the immediate future, and all the while the science behind 'T. sebesensis' remained extremely problematic and in need of swift rebuttal. Why? In short: none of us concerned with pterosaurs or European palaeontology want to deal with this outrageous, nonsensical claim in future publications. Hence, we fell back on using the published accounts of UBB ODA-28 to construct an argument against the pterosaur identification. Given that our authorship team has collectively amassed thousands of hours examining actual thalassodromid pterosaurs, as well as turtle plastrons, and how obvious the turtle affinities of the specimen are, this method seemed more than sufficient for the task at hand. Despite allegations from Codrea and Grellet-Tinner, these were not the actions of a team hastily assembling a rebuttal, but a collective of experienced individuals succinctly calling out obvious flaws in bad science.

So there we go: that's our side of that mistruth. Hopefully, that's the last we'll hear of 'T. sebesensis' around these parts, for there are much more interesting and exciting things to cover: palaeoart guides, Triassic fuzzy saltating xerocoles, dinosaur fat humps... all coming soon.

References

Codrea, V. A., & Grellet-Tinner, G. (2014). Reply to Comment by Dyke et al. on "Thalassodromeus sebesensis, an out of place and out of time Gondwanan tapejarid pterosaur" by Grellet-Tinner and Codrea (July 2014)" Gondwana Research. IN PRESS

Tuesday, 12 August 2014

Thalassodromeus sethi after some worms. Note: not a turtle. From Witton (2013)

Today sees the publication of an article challenging an exciting claim made in recent pterosaurology (Grellet-Tinner and Codrea 2014). If you missed it, the article concerned identifies a thalassodromid pterosaur in uppermost Cretaceous rocks of Romania and the erects a new species, Thalassodromeus sebesensis Grellet-Tinner and Codrea, 2014. At the centre of this is ODA-28, an (alleged) fragmentary cranial crest only fully exposed on one surface. None of this may not sound like a big deal, except that other thalassodromids - including the alleged sister species, Thalassodromeus sethi - are only known from the Lower Cretaceous Araripe Group of Brazil. T. sebesensis thus is about 40 million years out of time and thousands of miles out of place, and also occurring when azhdarchid pterosaurs basically represent the entire diversity of Pterosauria (Grellet-Tinner and Codrea 2014). Suddenly, the routine act of naming of a new animal is rewriting our understanding of pterosaur evolution.

There's more. Despite having only a scraps of bone to work with, Grellet-Tinner and Codrea (2014) suggested the T. sebesensis crest anchored muscles to form a ‘sizeable fleshy crest’, acted as a rudder in flight, that it somehow highlighted co-evolution between Romanian pterosaurs and angiosperms, and ecological segregation between azhdarchids and thalassodromids. All of these ideas are pretty radical in one way or another, especially considering the fossil material they are based on.

Blah blah blah… extraordinary claims, extraordinary evidence etc. When T. sebesensis was published it raised the collective eyebrows of pterosaur workers for all the wrong reasons. ODA-28 has no obvious ties to Thalassodromidae (or Thalassodrominae, if that’s how you roll - see Witton 2009), Pterosauria, or even to a cranial crest. Today, I and 19(!) other authors have said this in print (Dyke et al. 2014), noting that ODA-28 lacks any pterosaurian synapomorphies or even features typical of the group. As anyone who has handled pterosaur fossils can attest, pterosaur remains are distinctive at gross and microscopic level, and ODA-28 lacks any features expected in pterosaur bone (e.g. extremely thin bone walls separated by trabeculae). Any resemblance to the Thalassodromeus sethi holotype is entirely superficial, and shared characters between the two specimens - notably the ‘fossae’ at the base of the ‘T. sebesensis’ crest - are really incomparable on detailed examination. A clear lack of symmetry in ODA-28 shows it is not a medial skeletal element either, and thus not the cranial crest of anything. In short, cancel the text-book revisions: the temporal and palaeobiogeographical anomaly of ‘Thalassodromeus sebesensis’ is just a fairly major misidentification of a scrappy fossil (Dyke et al. 2014).

Is ODA-28 anything exciting at all? Well, not especially. The specimen is clearly a piece of turtle plastron, exactly matching the internal structure of the hypoplastron and xiphiplastron of the Maastrichtian, Romanian genus Kallokibotion (above, Dyke et al. 2014). The anatomy of Kallokibotion has been documented fairly thoroughly and known for about 100 years (e.g. Gaffney and Maylan 1992), allowing us to be confident in this identification. Ergo, 'T. sebesensis’ offers nothing other than new a piece of fossil turtle and a name for the Kallokibotion synonymy list.

In all, a bit of an anticlimax. How did our short paper end up with 20 authors? The response was started by experts in the terrestrial faunas of upper Cretaceous Romania, who asked me and the Natural History Museum’s Lorna Steel if we could contribute a few paragraphs targeting the flawed pterosaur identity of the specimen. While we were working, it emerged that pterosaur experts from Brazil were also planning a response. The editors of Gondwanan Research, who published Grellet-Tinner and Codrea (2014), understandably only wanted one response, so the two teams joined forces. By the time experts in turtles, Romanian fossils and pterosaurs were all on board, we ended up with a truly international background: the USA, UK, Brazil, Romania and France are all represented.

A final note: this is not the first time thalassodromids have been pulled to the top of the Cretaceous. Kellner (2004) and Martill and Naish (2006) argued that a partial skull and mandible from the Maastrichtian Javelina Formation of Texas represented a thalassodromid based on perceived similarities with the thalassodromid Tupuxuara. While others have argued against this idea (the mandible and premaxillary morphology are more similar to those of azhdarchids - Lü et al. 2008; Witton 2013) - these claims have not been met with a sledgehammer response because the suggestions are not unreasonable. Sure, I don’t think the Javelina material in question is thalassodromid, but I can see why others might. ‘T. sebesensis’ has been swiftly rebutted by a crowd of experts because the underlying science is so clearly bogus that all concerned with pterosaur and Romanian palaeontology wanted it’s impact nipped in the bud. A response from Grellet-Tinner and Codrea will be published soon, so we'll see what they make of our rebuttal. To end on a high: there are exciting pterosaur remains coming out of Romania, and some of them are in the review/publication system already. Hopefully, we'll have some news on these out soon.

Witton, M. P. (2009). A new species of Tupuxuara (Thalassodromidae, Azhdarchoidea) from the Lower Cretaceous Santana Formation of Brazil, with a note on the nomenclature of Thalassodromidae. Cretaceous Research, 30(5), 1293-1300.

Main site

Become a Patron

The art of this blog, in book form

About Me

Palaeontologist and palaeoartist, based at the University of Portsmouth and specialising in flying reptiles. Check out my work at MarkWitton.com, follow me on Twitter @MarkWitton, and buy my book here. Contact me about my work at wittonprints@gmail.com.